1. Field of the Invention
This invention relates to a circuit and method for controllably and independently recycling the discharge current of a plurality of inductive loads connected between first and second power supply terminals.
2. Discussion of the Related Art
Throughout the description to follow, the invention will be discussed, by way of non-limitative example, as applied to electronic systems specifically intended for automotive use which comprise devices having inductive loads connected in the manner mentioned above.
As is known, a number of electronic and electromagnetic devices, arranged to perform a variety of functions in motor cars, include one or more inductive loads. Such devices also distinguish themselves by the current that their respective loads are to withstand. Some inductive loads are of relatively small size and can only withstand moderately large currents during their operation. Devices of this kind may include relays of several different types, for example. On the other hand, there are inductive loads of larger size, such as those contained in fuel injection control devices, which can store larger amounts of energy.
In the systems referred to hereinafter, each inductive load usually has one terminal which is kept at a fixed voltage and another terminal connected to a preferably integrated control circuit driving the flow of current through the load.
Two configurations are known in the art wherein the fixed voltage terminal of the load is connected either to the power supply line or to ground. In the former case, the load is driven from its lowest voltage terminal, in a so-called low-side driver configuration, whereas in the latter case, the configuration is that known as high-side driver.
The control circuit comprises an active element, such as a driving transistor, which in most cases is a power transistor and performs switching functions to alternately force and interrupt the flow of current through the inductor. The active element is controlled by a drive signal generated within the circuit itself.
It is well recognized that the driving of markedly reactive loads, as inductive loads happen to be, generally causes some problems during the transients. In fact, upon cutting off the flow of current through an inductor, a voltage increase, i.e. an overvoltage, instantaneously appears across it which may be positive or negative depending upon the specific circuit design. This overvoltage is due to the energy previously stored in the inductor during the charging phase and generated by the flow of electric current forced through the inductor remaining constant even after the flow of current is cut off. Thus, an electromotive force is induced which tends to keep the current at the value attained during the "on" period, i.e. during the charging phase. Since the load has one terminal at a fixed potential, the increase in potential will occur at the other terminal, i.e. the terminal for connection to the control circuit.
For the device to operate as designed, the voltage peak should have limited width, for otherwise the peak could cause breakdowns in the junctions of the semiconductor elements present in the control circuit, or in other devices connected to this circuit. Where the circuit is integrated monolithically, the overvoltage may fire parasitic transistors, and ultimately cause the device to breakdown.
As is known to skilled persons in this art, to reduce the overvoltage a means may be provided for dissipating the energy stored up in the inductor, i.e. a means enabling a discharge current from the inductor to be recycled. The energy built up in the inductive load is dissipated by causing current to flow through pre-arranged elements, usually power ones.
In conventional approaches, the recycling circuit also functions to regulate the voltage increase. While the current is being recycled, the voltage on the inductive load reaches a maximum value which is set by the recycling means itself. This voltage limiting effect is called "clamping" in the pertinent literature. The value of the clamped voltage at that maximum remains constant through a given time interval, to then decrease in absolute value, simultaneously with the current, down to a zero value which corresponds to a fully discharged inductive load.
The duration of the discharging phase, i.e. the so-called discharge time, is tied directly to the maximum voltage value reached on the inductor.
The discharge time is of particular consequence. It is frequently important that the discharge time be controlled accurately. For example, with devices for automotive use, it is on the basis of this time that logic control circuits operatively connected to the devices are correspondingly set. In addition, if the device is a fuel injector which is controlled to open and close by the presence or absence of a current flow through the load, it is important that the control current flowing through the load can be cut off within a short time, so that the closing time of the injector can be made short and a precise amount of fuel injected.
The discharging phase is regulated by suitably sizing the recycling circuit to correspondingly select the maximum voltage value.
It is convenient if this regulation can be performed substantially independent of changes in outside conditions, such as ambient or fabrication temperature, or process spread in the instance of integrated circuits.
In accordance with the prior art, a well-known class of circuit designs, to which this invention is related, have been provided to recycle the current through the same transistor which is to drive the inductive load and already arranged to accept the discharge current peak through it, this usually being a power element. A recycling regulating circuit is connected between the load terminal arranged to be connected to the control circuit, that is to the driving transistor, and a control terminal of the transistor. That circuit will control the value of the voltage present on the load and automatically turn on the transistor when this voltage reaches a predetermined maximum value. In the field of automotive devices, this voltage maximum value is commonly on the order of a few tens volts, a typical value for a fuel injector being 70 volts, for example.
Those circuit elements which define the maximum voltage value represent, within the recycling regulating circuit, what throughout the following description will be referred to as the "reference for the voltage".
A variety of conventional designs exist in this field.
A first solution provides one or more Zener diodes connected between the control terminal of the driving transistor and the inductive load. The Zeners constitute the reference for the voltage in this case, the maximum voltage being a combination of the Zener voltages.
Shown diagramatically in FIG. 1 is one such circuit. In this example, the inductive load L is connected in a low-side driver configuration, it having one terminal connected to the supply line Vs and the other terminal to the drain terminal D of a MOS driving transistor, here an N-channel type denoted by TM. The transistor TM has its source terminal connected to ground. Connected to the gate G, forming the control terminal of TM, is a driver circuit schematically illustrated by the block C. Connected in series between the gate G and the node D, are a number, n, of Zener diodes schematically indicated at nZ in the Figure. The Zeners are configured to set the maximum voltage at the node D to a value Vmax=nVz+VGS, where Vz is the Zener voltages and VGS is the drop in potential across the gate and the source of the transistor TM. As, upon the transistor TM being turned off, the voltage at the node D attains the value Vmax, the transistor TM will go into conduction, thereby allowing the current to be recycled and discharging the inductor L.
A drawback of this circuit is that the maximum voltage value on the inductor, as determined by the combination of a discrete number of Zener voltages, may not equal exactly the desired one. In the instance of an integrated circuit, it should not be overlooked, moreover, that if the maximum voltage value is fairly high, then the number of the Zeners and the area occupied by the recycling regulating circuit is greater than negligible. The voltage value of the Zener diodes also is liable, as is well recognized, to be indeterminate due to significant process spread.
Another prior circuit solution, which represents a substantial improvement on the first, is shown in FIG. 2. This provides one or more additional transistors in the recycling circuit.
In FIG. 2, corresponding elements are denoted by the same references as in FIG. 1. The recycling regulating circuit further comprises at least one transistor Q of the bipolar type, connected between the gate G and the node D. The Zener diodes nZ are here connected between the base and the collector of the transistor Q. The maximum voltage at the node D is set to the value Vmax=nVz+VBE+VGS, where VBE is the drop in potential between the base and the emitter of the transistor Q. Thus, the maximum clamping voltage will be the resultant of a combination of a larger number of voltage drops than in the circuit previously described. The transistor Q functions to absorb possible excess currents and prevent the Zener from being burned. The resistor R, depicted in series with the Zener diodes in FIG. 2, is also effective to limit the current.
Neither of the circuits just described has good stability. In fact, since the voltages across the Zeners have different dependence on temperature, temperature compensation can only be achieved for certain definite maximum voltage values.
For the purpose, the prior art provides still more sophisticated and precise solutions. For example, one conventional circuit design provides for the use of a recycling regulating circuit which includes a voltage divider and a comparator. A circuit of that kind is disclosed in European Patent Application EP-0622717.
Such a circuit is illustrated in schematic form by FIG. 3. The voltage divider R1-R2 is connected between the inductive load L and a ground terminal. The comparator COMP has two inputs respectively connected to the output of the voltage divider and to a generator of a reference voltage Vref, and has an output coupled to the gate terminal G of the driving transistor TM. In the example of FIG. 3, the circuit further comprises a current mirror M having an input leg connected to the output of the comparator COMP and an output leg connected to the gate terminal G. The mirror M is also supplied to the node D.
Basically therein, the reference voltage is compared with a divided value of the voltage to be regulated which is present on the node D. Current recycling takes place upon the voltage value Vmax=Vref(R1+R2)/R2 being reached on the node D. The reference for the voltage comprises that voltage reference circuit and the divider. Since the variation in the maximum voltage depends on a voltage reference which can be easily selected to be a stable one temperature-wise, that recycling regulating circuit is quite stable temperature-wise.
The systems to be specifically discussed in connection with the invention include, as previously mentioned, a plurality of inductors. For example, in the instance of plural automotive devices wherein the different inductive loads require driving on an individual basis and must be operated each independently of the others, the recycling of the discharge current from each load should take place at different times from the other loads, and the discharging time required may not be the same for all the loads.
It will be appreciated that the principles of this invention can also be applied to devices of a different kind, e.g. those used in industrial systems employing multiple stepper motors.
To provide independent recycling for different inductive loads, it has been common practice to recycle current through a plurality of recycling regulating circuits, each connected to a respective one of the loads. The overvoltage clamping is done with a separate reference for each load.
The reference for the voltage may be provided in any desired way. In general, circuits incorporating Zeners like those shown in FIGS. 1 and 2 are used. However, the combination of a number of such circuits, of the types previously described, has some disadvantages. The overall number of Zeners employed actually tends to be fairly large. To ensure restoration of a plurality of driving transistors to the on state in an independent manner, it is necessary that a separate series of Zeners be connected to each transistor, so as to directly limit the voltage to the respective node D for connection to the load and leave the operation of the other transistors unaffected. However, the number of the Zeners may be of about ten in each series, at the aforementioned maximum voltage values, which means that a very large area of the integrated circuit will be occupied.
Even where recycling circuits of the type shown in FIG. 3 are used, in order to ensure independence of recycling for the various loads, because it would be disadvantageous to uncouple the renewed turning on of the driving transistors, and hence the recycling, in any other ways, a corresponding number of dividers and comparators must be provided, which again poses problems of area occupation.
A technical problem that underlies this invention is to provide a circuit for independently recycling a plurality of inductive loads connected between two power supply terminals, which circuit can be integrated monolithically within a small integration area.
An object of this invention is to provide a recycling circuit which is stable to variations in ambient conditions, such as temperature, and to varying parameters during its manufacture.